Diclofurine is a new ketone-oxime derivative with potent peripheral vasodilating properties. Its hypotensive effects were studied in 24 patients suffering from severe hypertension resistant to combinations of most hypotensive drugs. In 22 patients who received diclofurime alone the blood pressure decreased from mean values of 194 +/- 1/118 +/- 3 to 166 +/- 4/97 +/- 3 mmHg after the weeks of treatment and remined at simular levels on further examinations. Most remarkably, the fall in BP was accompanied by a decrease of 15 beats/minute in supine heart rate. The mean daily dosage of diclofurime was 518 mg, the drug being taken in three divided doses. In 7 cases where diclofurime was combined with acebutolol either from the start or (in 5 cases) after an initial period of monotherapy, the efficacy of the treatment was enhanced by the beta-blocking agent. Out of 24 patients studied only one failed to respond. The new drug was generally well tolerated. Diclofurime appears to be one of the most active and best tolerated drugs for long-term oral treatment of arterial hypertension.